The present invention relates to an image signal processing circuit and, particularly, to such circuit by which the noise level of an image signal in a VTR or television system is reduced.
FIG. 1 is a block circuit diagram showing a conventional image signal processing circuit, which is shown in Japanese Patent Application Laid-Open No. 130835/1979. In FIG. 1, reference numeral 1 depicts an input chroma signal, 4 a 1H delay line for delaying the input chroma signal by a time corresponding to one horizontal scanning period (1H), 5 an output of the 1H delay line 4, 12 an adder for summing the output 5 of the 1H delay line 4 and the input chroma signal 1, 13 a vertical high frequency component which is an output of the adder 12, 14 an amplitude limiter for limiting the amplitude of the vertical high frequency component 13, 15 a vertical noise signal, 16 a level regulator circuit for regulating the level of the vertical noise signal 15, 17 a phase inverter for inverting the phase of an output signal of the level regulator circuit 16, 20 an adder for summing the output signal of the phase inverter 7 and the input chroma signal 1 and 21 an output chroma signal.
In operation, the input chroma signal 1 is delayed by the 1H delay line 4 and is summed with the delayed output 5 from the delay line. That is, the 1H delay line 4 and the adder 12 constitute a comb-type filter, an output of which is the vertical high frequency component 13 in a vertical space frequency range.
A high-level signal component of the vertical high frequency component 13 is cut out by the amplitude limiter 4, resulting in the vertical noise signal 15 composed of only a low-level noise component. The level of the noise signal 15 is regulated by the level regulator 16 such that it is substantially the same as the level of noise contained in the input chroma signal 1, and then the phase thereof is inverted by the phase inverter 17. The output signal of the phase inverter 17 is summed with the input chroma signal 1 by the adder 20, resulting in the output chroma signal 21 having noise components removed.
The comb-type filter shown in FIG. 1 may be replaced by a feedback-type comb filter with the same effect.
In the conventional image signal processing circuit, particularly such a circuit using a recursive comb filter whose phase characteristic is not linear, there may be a case where a signal appears in a place in which such signal should not exist or no signal appears in a place in which such signal should be present. Therefore, if such circuit is used in an automatic phase control loop (APC loop), the latter may malfunction.
FIG. 2 shows an APC loop for a VTR (Video Tape Recorder) in which an image signal processing circuit having the feedback-type comb filter is employed. The APC loop functions to restore the original frequency range (3.58 MHz) of the input chroma signal from a lowered frequency (629 KHz) thereof using an up-converter 30. That is, the color burst signal derived from the output chroma signal by a burst gate 31 is compared in phase with a reference signal (3.58 MHz) from a crystal oscillator 32 by a phase detector 33, and the output of a voltage controlled oscillator 34 is controlled to 3.58 MHz+629 KHz by locking phases. However, since the recursive comb-type filter functions to average the phase variations of the output chroma signal, the locking speed of the APC loop is lowered thereby when a phase locking state is to be reestablished after it has once been unlocked. Similarly, there may be noise generated due to the transient response of the circuit in a high speed picture search mode of the VTR. Furthermore, in the described circuit in which the signal amplitude is limited by the limiter 14, all signal components other than noise are not removed. Therefore, not only the noise, but also high frequency components of the input chroma signal may be removed during the processing of the remaining signal components in the adder 20. Consequently, degradation of the image quality due to degradation of the vertical resolution may be increased, rather than the desired effect of noise removal, depending upon the mode (particualrly, in the standard reproduction mode).